1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus having a charging roller, image transfer member or similar contact member which is applied with a voltage in contact with a photoconductive element or similar image carrier, with or without the intermediary of paper. The present invention further relates to the use of a temperature sensor which is used in both contact and separation positions relative to an object whose temperature is being detected.
2. Discussion of the Background
Generally, an image forming apparatus of the type described, e.g., a facsimile apparatus or a printer includes a charging device for charging a photoconductive element or image carrier, and an image transfer device for transferring a toner image from the photoconductive element to a piece of paper. The charging device and image transfer device have often been implemented by a corona discharger having a discharge wire made of tungsten and not contacting the object to be charged. The charging device implemented by a corona discharger has the following problems.
(1) A voltage as high as 4 kV to 8 kV has to be applied to the discharge wire in order to deposit a charge potential of 500 V to 800 V on the photoconductive element,
(2) Since most of the current from the discharge wire flows into a shield, only several percent of the total discharge current is available for charging the surface of the photoconductive element to the predetermined potential, obstructing efficient use of power.
(3) Corona discharge ionizes the air and generates a great amount of ozone, nitrogen oxides and other harmful substances. To prevent such substances from deteriorating the parts of the apparatus and the surface of the photoconductive element, the apparatus has to be provided with an ozone filter, a fan for generating a stream of air, etc.
(4) Images are apt to become irregular due to the contamination of the discharge wire.
In light of the above, there has been proposed a charging device having a charge roller or similar charging member which charges the photoconductive element in contact therewith when applied with a voltage. Such a contact type charging device is advantageous over the above-stated non-contact type device, as follows. The device reduces the voltage necessary for the predetermined charge potential to be deposited on the surface of the photoconductive element. The device produces a minimum of ozone during the course of charging and, therefore, eliminates the need for an ozone filter while simplifying an exhaust arrangement.
However, the problem with the contact-type charging device is that the charging efficiency, i.e., a ratio of the charge potential to the applied voltage, changes with a change in the surface temperature of the charge roller; the former decreases with a decrease in the latter. It follows that in the case of constant voltage control, a decrease in charging efficiency lowers the charge potential and, therefore, image density for a given applied voltage. In addition, the other process control, also using the charge potential as a reference value, becomes faulty.
To eliminate the above problems, Japanese Patent Laid-Open Publication No. 4-6567, for example, proposes an arrangement wherein the charge roller or similar charging member itself is heated to 35.degree. C. to 55.degree. C. so as to obviate defective charging even in a low temperature environment. To heat the charging member, a heat source is disposed in or in the vicinity of the charge member, or heat from a fixing device is fed to the charging member. For temperature adjustment, use is made of a thermostat or similar conventional temperature adjusting member.
By so controlling the temperature of the charge roller or similar contact member contacting the photoconductive element, it is possible to maintain a charge potential which does not degrade images. However, the heat heats not only the charging member but also the photoconductive element and other process units adjoining the heat source. As a result, toner collected from the photoconductive element after the image transfer is heated while it is returned to a developing device. This brings about so-called toner blocking and aggravates the cohesion of toner.
Japanese Patent Laid-Open Publication No. 4-186381, for example, teaches an improved charging device having a temperature sensor directly contacting the charge roller. In response to the output of the temperature sensor representing the surface temperature of the charge roller, the voltage to be applied to the roller is controlled to deposit a stable charge potential on the photoconductive element. This successfully eliminates the problems discussed above in relation to Laid-Open Publication No. 4-6567. In addition, since the temperature sensor directly contacts the charge roller, it can sense the surface temperature without regard to the ambient atmospheric temperature and, therefore, insures an adequate voltage.
However, even the charging device using a temperature sensor as stated above has some problems yet to be solved, as follows. Although the contact type charging scheme reduces the voltage required of the charge roller, compared to the non-contact type scheme using a corona discharger, a voltage as high as 1 kV to 2 kV is still necessary and affects the temperature sensor and other constituents in various ways.
For example, when such a high voltage is applied to the charge roller, electric noise is apt to enter a control circuit, which controls the voltage to the charge roller, via the sensor contacting the charge roller. Moreover, short-circuiting is apt to occur due to a small breakdown voltage. This causes the control system to malfunction or, in the worst case, breaks it. Further, the sensor contacting the charge roller causes the roller to wear, causes toner and paper dust and other impurities to adhere to the roller, and produces noise while the charge roller rotates in contact with the sensor. Although these problems may be eliminated if the sensor is spaced apart from the charge roller, then the sensor fails to sense the surface temperature of the roller with accuracy.
The foregoing description has concentrated on a charge roller which is applied with a voltage in contact with a photoconductive element. However, it is also true with an image transfer roller which is applied with a voltage in contact with a photoconductive element with the intermediary of a paper. Specifically, in the case of constant voltage control, if the surface temperature of the image transfer member is low, a toner image cannot be efficiently transferred from the photoconductive element to the paper.